Electron emission cathodes, also referred to as electron emitters or electron sources, are used in devices such as scanning electron microscopes, transmission electron microscopes, semiconductor inspection systems, and electron beam lithography systems. In such systems, an electron source provides electrons, which are then guided into an intense, finely focused beam.
One type of electron source widely used in modern electron beam systems is the thermal field emission cathode, which uses a combination of heat and electric field to emit electrons. One type of thermal emission cathode is a Schottky emission cathode, commonly referred to as a Schottky emitter. (Although the term “Schottky emission” refers to a specific operating mode of an emitter, the term “Schottky emitter” is used more broadly to describe a type of electron emitter that may be capable of operating in a variety of modes, including a Schottky emission mode.) A Schottky emitter uses a very thin coating on a heated emitter tip to reduce its work function, that is, the energy required to free an electron from the emitter surface.
FIG. 1 shows part of a typical prior art Schottky emitter 12, such as the one described in U.S. Pat. No. 3,814,975 to Wolfe et al. for “Electron Emission System.” Schottky emitter 12 includes a polycrystalline tungsten, hairpin-shaped filament 14 that supports and heats an emitter 16 having an apex 22 from which the electrons are emitted. Applicants herein use the term “emitter” alone to refer to that portion of the electron source from which electrons are emitted (e.g., emitter 16 of FIG. 1) and the terms “Schottky emitter” and “thermal field emitter” to refer to the entire electron source assembly (e g., Schottky emitter 12). Heating current is supplied to filament 14 through filament posts 26, typically composed of molybdenum, kovar, or tungsten and extending through both sides of a base 28. Filament posts 26 are typically inserted through close-fitting holes 30 in base 28 and secured by brazing. Schottky emitter 12 typically operates with apex 22 at a temperature of between 1,700 K and 1,900 K, most typically at around 1,800 K. A suppressor cap 32 is typically press fitted onto base 28 and extends out to near the emitter apex 22 to reduce the undesirable emission of electrons from the shank of the emitter.
Emitter 16 is typically made from a single crystal of tungsten oriented in the <100> direction and coated with a coating material, such as zirconium and oxygen, to lower the work function of the emitter tip by approximately 1.5 electron volts. At the high temperatures at which Schottky emitter 12 operates, the coating material tends to evaporate from emitter 16 and must be continually replenished to maintain the lowered work function at apex 22. A reservoir 34 of the coating material is typically provided to replenish the coating on emitter 16. The material from reservoir 34 diffuses along the surface and through the bulk of emitter 16 toward apex 22, thereby continually replenishing the coating there. The input power required to heat a Schottky emitter is substantial, typically somewhat greater than 2 watts.
As electron beam instruments become more accepted in production environments as inspection and processing tools, users demand increased throughput. One method of increasing speed entails incorporating several electron beams into a single system. In such systems, the heat conducted and radiated from multiple thermal field emitters is additive and could produce an overall system temperature that would be unacceptably high and can cause drift in the emitter position. Another trend in electron beam instruments is miniaturization. Smaller instruments cost less to construct, take up less space in a production area, and are more mobile. Smaller instruments are particularly well suited for production applications, such as electron beam lithography for forming microscopic structures in integrated circuit assembly. For example, U.S. Pat. No. 6,218,664 to Krans et al. describes an electrostatic objective lens and electrical scanning device that can be used in a very small electron beam system.
It is desirable to reduce the size and power consumption of thermal field emitters to allow construction of smaller electron beam systems and to fit more electron columns within a system requiring the use of multiple emitters. Because of the small volume and dense packing of components in such systems, it is desirable to reduce not only the size of the thermal field emitter, but also the power consumption and the heat output of the thermal field emitter or emitters. The great amount of heat produced by the multiple closely packed thermal field emitters can adversely affect the operation of the electron columns.
One method of reducing the undesirable heat conducted and radiated into the electron column from a thermal field emitter is to use a substantial quantity of a thermally conductive material attached to the base and filament posts to provide a path to conduct excess heat away from the source. Unfortunately, cooling the base or the filament posts tends to cool the emitter assembly as well, and so has the undesirable effect of increasing the input power required to maintain emitter apex 22 at an acceptable operating temperature.